A buck boost converter is a type of DC-DC converter that is capable of providing a positive output voltage that is higher or lower than its input voltage. As battery powered devices are becoming more and more popular, this topology is becoming more attractive as it can make use of the discharge cycles of a battery. When a battery input voltage is higher than its output voltage, a buck boost converter works in the buck mode of operation. In the buck mode of operation, the converter decreases the input voltage to a suitable level for use at its output. When the battery input voltage is lower than the output voltage, the buck boost converter works in the boost mode of operation wherein the input voltage is increased to a suitable level at the output.
It is relatively easy to implement the control in either a pure buck mode of operation or a pure boost mode of operation by leaving some power switches turned on or off. The challenge remains in the transition between the buck and boost modes of operation when the output voltage is close to the input voltage. In the buck-boost mode transition, a voltage gap is generated between a boost ramp valley and buck ramp peak. In the voltage gap area, a buck cycle of the buck-boost mode runs in maximum duty and a boost cycle of the buck-boost mode runs in minimum duty. Generally, there is no duty cycle modulation to overcome an input voltage change in the voltage gap area, and thus, the output voltage generally follows the input voltage, which leads to poor line transient response.